1. Field of the Invention
This invention relates to a novel coating method and a novel coating apparatus useful for practicing the same, more particularly to a coating method which is suitable for applying a coating solution such as one for forming a magnetic recording medium which is of relatively high viscosity.
2. Description of the Prior Art
The term "web" as used herein is not limited and is intended to mean a relatively long, flexible, belt-shaped support such as: a plastic film of cellulose triacetate, polyamide, polyimide, polycarbonate, polyethylene terephthalate or polyvinyl chloride; paper; synthetic paper; a metal foil of aluminum, or copper; and a sheet of glass or ceramic and the like.
The term "coating solution" as used herein is not limited and is intended to mean a variety of coating solutions different in composition depending on the purposes of use. Examples of such "coating solutions" are coating solutions for forming a photosensitive emulsion layer, a base layer, a protective layer and a back layer as are conventionally used in a photographic photosensitive material; coating solutions for forming a magnetic layer, a base layer, a wetting layer, a protective layer and a back layer as are conventionally used in a magnetic recording medium; and coating solutions for forming an adhesive layer, a coloring layer, a rustproof layerings, etc. These coating solutions are made up of essential components, a binder, and, if desired or necessary, water or an organic solvent or dispersion containing various additives.
In order to form a coated film on the above-described web to a desired thickness, the following methods are now extensively employed: Conventional coating means such as a dip, reverse roll, gravure roll, extrusion hopper or slide hopper are used to apply an excess of coating solution to a web which is run continuously. Then, a metering means such as an air knife, blade or coil bar (or wire bar) is contacted with the coated layer to scrape off surplus coating solution to coat the web with the coating solution to the desired degree or thickness.
In coating a web with a coating solution of high viscosity such as a magnetic solution for forming a magnetic recording medium, a coil bar has been extensively employed as the metering means because it is simple in structure and can be readily handled as compared with an air knife or blade, and the coated surface is relatively stable in quality.
A conventional coating method using a coil bar is shown in FIG. 1.
As shown in FIG. 1, a web 1 is run continuously in the direction of the arrow, and a coating solution 3 is applied to the lower surface of the web 1 at a thickness larger than finally required to form a coated film 5 in the liquid state. Surplus coating solution 7 is scraped off by the coil bar 6 so that a coated film 8 is formed which is regulated by the gap which is defined by the cylindrical wall of the coil bar 6 and the surface of the web 1. If desired or necessary, for instance in the case of forming a magnetic layer, the same can be subjected to magnetic field orientation and to drying, and then wound. In FIG. 1, reference numeral 4 designates guide rolls rotating in the direction shown by the arrows.
Coil bar 6, as shown in FIG. 2, is made up of a core material or a rod member 9 and a wire 10 which is wound in the form of a single coil on the cylindrical wall of the rod member 9 with the turns being in close contact with each another. In general, the rod member 9 is a stainless steel, iron or brass rod 1 to 3 mm in diameter and the wire 10 is a stainless steel or "Teflon" wire 0.04 to 0.05 mm in diameter.
It is necessary to smooth the surface of the coated film or coated layer to various degrees depending on the purpose of use. After being subjected to metering with the coil bar, the coated film or coated layer is smoothed by suitable means.
In such a conventional coating method, the web is left in a gaseous circumference during the time interval from the time that the coating solution is applied to the web until the metering operation is carried out and during the time interval which from the instant that the metering operation is effected until the smoothing operation is carried out. During these time intervals the coated film is not yet dried and not yet hardened, and, therefore, the metering or smoothing operation is liable to be affected by the viscosity of the coating solution, interfacial tension, and web velocity.
During metering or smoothing air may enter between the metering member and the coated film or coating material which has been solidified on the metering bar may mix therewith, thus harming the coated surface. This tendency becomes significant as the viscosity of the coating solution increases and coating velocity increases. Therefore, rib-like (streak-like) stripes are formed on the coated surface, thus degrading the smoothness of the surface.